Air Assist on Front Forks, VL800 - Page 3 - Suzuki Volusia Forums : Intruder Volusia and Boulevard Forum
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post #21 of 54 (permalink) Old 10-10-2013, 11:31 AM Thread Starter
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I wouldn't recommend running 200 anyway as I suspect it would ride like there was no front suspension. At 60 P/SI it's impossible to compress the fork...well at least with my 260lbs pushing straight down on it anyway....no give at all, that i could detect.

Roughly knowing the upper limits is for determining how much of a safety ceiling is available above more realistic pressures before you need to be concerned about having a catastrophic failure.

I'd expect with no spring in, the upper comfortable ride/ good handling limit would be around 60-70 P/SI.

Which for comparison is 50-40 P/SI below the pressure in my racing bicycle's ultra thin skinned super narrow tires. (110 P/SI)

80 is dead on for my '71 Schwinn 10 speed's tires which are also very thin skinned. But I think the surface area of the contact patch for it is very telling. For the '71 the contact patch of both tires (occasionally only one tire bearing the full load ) supporting about 290lbs of rider and bike is about the size of a quarter per tire, and yields a very bone shaking ride on anything but the smoothest of pavement. You literally feel every pebble and crack.

A quick look at the Vol's builder's plate shows GVW to be 1025lbs, GAWF 355lbs, GAWR 670lbs.

To put that into perspective our fork tubes have more than 100% greater surface area ( 982mm\2 VS 2267.08mm\2 ) at the ends supporting a recommended maximum of 55lbs more weight. (wonder what the wheels and tires weigh, I also wonder if those weights are at the axle including wheel and tire weight or not. If they include wheel and tire I suspect there is close to 30lbs less weight on the fork at maximum allowable load, not including the weight of the sliders which are also not part of the weight carried by the springs. and therefore would result in roughly only 320lbs being borne by the upper suspension).

So in short anything over 80 P/SI is very likely serious overkill, and very likely going to negatively affect comfort and handling. 80 P/SI yields 283lbs of force required before the fork even attempts to begin to compress. Of course as it compresses air pressure increases requiring increasingly more force required to compress further.

Using the realistic estimate of 200 as the likely to fail limit leaves a ceiling of 120P/SI to account for a good safety margin. To run any pressure vessel at it's working limit is foolish at best (despite the failure rating being typically 150%-200% of the working pressure).....just like running an engine at it's mechanical limits. Sooner rather than later it will fail.

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post #22 of 54 (permalink) Old 10-10-2013, 12:25 PM Thread Starter
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I still need to measure the stroke and determine the overall change in volume to determine the ratio of shocks fully extended to fully compressed to determine the maximum pressure the forks would see were they to bottom out.

An 8:1 compression ratio in an engine should yield around 118 P/SI while the change in volume in our forks likely changes by less than 2:1 at most or around 30 P/SI. which would put the fork at 110 at full compression starting at 80 at full extension.
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post #23 of 54 (permalink) Old 10-10-2013, 05:02 PM Thread Starter
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50 miles on air only (springs are in a garbage bag in the bed of my truck), 70P/SI too soft for really really hard braking, but I've not updated the retainer clip yet so I ain't going higher for now.

Found a really really really huge problem with doing this.............................................. ...


The bike looks really f'n good with the forks completely compressed....too f'n good...now I need to sort out a means of....well, I dunno just yet, but lowering the bike at both ends is now super high on my list of things to do.
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post #24 of 54 (permalink) Old 10-11-2013, 01:07 PM
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These forks are not designed to hold air pressure.
Of course you get some when they are compressed, but only a few psi.
Your going to be hurdling down the freeway at 70 and one of your seals are going to blow and you going to be in a lot of trouble.

I've had motocross bikes that take a bit of air on top of the spring, but usually only 2-7 psi, and the seals on those types of forks are considerably different than these.

I have also had bikes with just-air springs that work on bladders and such.

I have major reservations about putting air-only into a suspension system that is not designed for any air compression at all.

Why use a big word when a diminutive utterance would suffice?
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post #25 of 54 (permalink) Old 10-11-2013, 11:29 PM Thread Starter
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For not being designed to hold pressure they sure are airtight. One full tank burned through without any new issues. Contemplating reinstalling the springs albeit after sawin off enough coils to allow the front to ride a bit lower when there is no pressure in the forks...maybe an inch or two up from completely compressed...needs to dig out my lowerin bones...yeah she'll scrape if I push er but she'll look soooo f'n good...methinks my ratbike days are numbered, with a push towards more aggressive stylin, clean paint, and cruddless brightwork on the horizon.
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post #26 of 54 (permalink) Old 10-12-2013, 01:29 AM
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Quote:
Originally Posted by dfinitlydisturbd View Post
I still need to measure the stroke and determine the overall change in volume to determine the ratio of shocks fully extended to fully compressed to determine the maximum pressure the forks would see were they to bottom out.

An 8:1 compression ratio in an engine should yield around 118 P/SI while the change in volume in our forks likely changes by less than 2:1 at most or around 30 P/SI. which would put the fork at 110 at full compression starting at 80 at full extension.
OK, small physics point here:

P2 = P1 x V1/V2 is Boyle's law. So the final pressure (P2) is the initial pressure multiplied by the volume ratio. However, the pressures are measured in ABSOLUTE pressure, not Gauge pressure, so you need to add 1 atmosphere (14.7psi) to your measured pressures.

So in your case, P1 = 80 psi PLUS 14.7 psi = 94.7 psi absolute

At 2:1 volume ratio, P2 = 94.7 x 2 = 189.4 psi (ABSOLUTE), so on the pressure gauge you would read 189.4 minus 14.7 = 174.7psi

But it's not as simple as that! The above calculation assumes that the compression happens s-l-o-w-l-y. In practice, at high speed the air inside actually heats up when it is compressed, and that INCREASES the pressure further. In this case it would go to around 218psi.

The compression pressure vs compression ratio of an engine is not relevant here, because it is influenced by valve timing.

So, the final pressure is very dependent on the the volume ratio of your forks. You need to measure it, or calculate it accurately. And a word of warning, remember to allow for the volume taken up by the damping oil in the forks.

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post #27 of 54 (permalink) Old 10-12-2013, 12:07 PM Thread Starter
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F'KING SITE!!! Why is it only when I have a lot of time invested in a post and forget to copy it to notepad first that it decides to forget I'm logged in! I thought that was the whole frigging point of clicking remember me every single f'n time I log in.

Oh well, I ain't wasting time fully typing it out again so here's the abbreviated version.
Fork volume inc. oil = 150.99645499 cl
Fork volume inc. oil compressed = 99.070765294 cl
Temperature difference caused by compression and expansion = 39.31 deg.F.
Starting P/SI = 80 @ 75 deg. F.
Ending P/SI = 123 @ 114 deg. F.

WOW! Twice in a row a new friggin record. at least I saved it to notepad this time

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post #28 of 54 (permalink) Old 10-12-2013, 12:15 PM Thread Starter
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Apologies to brg, I had a much more eloquent post prepared, but it is now lost. Am too aggravated to even think about preparing another. Tis not the first time here and certainly will not be the last that a well prepared post vanished thanks to the board automagically logging users out while they're posting.

For those following it's still holding at 80, no leaks, no catastrophic failure yet.

As always will keep you updated on issues, changes, modifications, etc.

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post #29 of 54 (permalink) Old 10-12-2013, 09:24 PM
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Quote:
Originally Posted by dfinitlydisturbd View Post
....
Fork volume inc. oil = 150.99645499 cl
Fork volume inc. oil compressed = 99.070765294 cl
Temperature difference caused by compression and expansion = 39.31 deg.F.
Starting P/SI = 80 @ 75 deg. F.
Ending P/SI = 123 @ 114 deg. F.
....
I haven't experienced problems with losing posts, but that's probably because I'm new, and haven't done many.

So, the 151cc is the air volume (above the oil) before you compress the fork, and 99cc of air left after you compress, yes? That's a 52cc volume change. That sounds small. The forks are 40mm dia (I think) and the travel is 140mm according to my manual, so that's 176cc of volume change. Something doesn't add up. Have I misread your figures?

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post #30 of 54 (permalink) Old 10-13-2013, 03:26 PM Thread Starter
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Don't forget we're dealing with inside dimensions not outside. Total travel of the fork from extended to fully collapsed is 5" (number obtained by measuring both states and subtracting). I really wish the site hadn't managed to lose my post or at the least I had done the usual copy and paste. It contained all dimensions instead of just the ones I provided above.

I'll go back through and regather the missing info for repost.

UPDATE:
cut the springs down at the tight end and reinstalled. Now have one inch of travel on the springs at no pressure. At 30P/SI I have about two and a half inches and the bike sits and rides well....though the effect on steering geometry can be felt, the bike is twitchy and reacts to steering input a bit quicker than I like...steering is too light for what I'm used to, but because the bike looks good lowered I believe I'll just adapt, should correct when I lower the rear. It's weird, I actually felt the rear lift a lot (definitely much higher than usual) on a couple aggressive stops while seeing if I could bottom out the forks.
Interesting. In the meantime the bike rides well even with zero pressure, has no issues at 70 P/SI (other than riding at stock height and not being as pretty). Still haven't updated the retaining clips and need to investigate a rebound spring that supposedly lives under a retainer in the bottom of the fork as there is unexplained additional fork travel just before 100 and continuing until 160 P/SI that is too far to be attributed to the seal flexing.
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